1. Field of the Invention
This invention pertains to components of a high bit-rate monomode optical communication system present in a digital transmission system. More specifically, it pertains to optical channel waveguides in planar lightguide circuits which can propagate and process temporal optical soliton signals prior to their entrance to, or at the exit from, an optical fiber.
2. Prior Art Statement
In a modern optical communications system there are two aspects which limit performance. The first of these is optical attenuation due to absorption or scattering by the transmission medium. Attenuation limits how far a signal can travel in an optical fiber transmission line before it requires amplification. The second aspect is signal pulse broadening due to dispersion, which limits the bit-rate, or bandwidth, because of a loss of detector discrimination between adjacent pulses in a communication bit stream.
The aim of the present invention is only digital signal systems in which the signal consists of the presence or absence of pulses within a pulse-bit stream. It is not concerned with analog systems in which the signal consists of a varying amplitude of an electromagnetic wave.
The current practice for long distance optical communications systems requires the use of "repeaters" which involves two optoelectronic conversions. Photons of an optical signal travelling in the optical fiber are converted, through use of a photodetector, to an electric signal, i.e. electrons. The signal is electrically amplified to correct for absorption and electrically reshaped to correct for dispersion and the resulting signal converted back to photons, e.g. through use of a diode laser, for transmission through the next optical fiber link.
Recently, however, erbium doped optical amplifiers have been implemented into some fiber optic transmission systems. This innovation has the marked advantage that amplification, to correct for attenuation losses in long distance systems, occurs without the need to convert to electrons. The second problem, signal dispersion, is not addressed by these optical amplifiers.
Clearly there is considerable technological, as well as commercial, advantage in eliminating the periodic repeaters still required in an optically amplified fiber optic communication system to reshape signals which have broadened through dispersion. Long distance or high bit-rate digital communications applications would benefit from an optical system in which no signal pulse broadening due to dispersion occurs.
Dispersion, which leads to pulse broadening, has two components. The first is material dispersion which is a bulk property of the waveguide material system and its composition. The second is termed waveguide dispersion. It is a function of the waveguide's geometry, its dimensions and the profile of the material composition within the waveguide. Taken together the two components are generally termed chromatic dispersion.
To transmit signals over long distances or for high bit-rate transmission, in general, it is necessary that a pulse does not change shape with time. This in turn requires that there be a way to compensate for the naturally occurring pulse broadening due to chromatic dispersion within the optical transmission system.
Hasegawa, U.S. Pat. No. 4,406,516, discloses that a solution to this dispersion problem lies in a fiberguide communications system that propagates temporal optical solitons as the digital signal. A temporal optical soliton occurs when the pulse broadening due to chromatic dispersion is balanced by the contraction due to a nonlinear dependence of the transmission medium's index of refraction on light intensity. In '516 the conditions necessary to achieve a fiberguide communications system which can propagate temporal optical soliton pulses is disclosed.
Hasegawa and Kodama, U.S. Pat. No. 4,558,921, disclose a repeaterless optical fiber communications system in which soliton pulse attenuation is non-electronically amplified by appropriate amounts at appropriate intervals. Refinements in this basic repeaterless soliton communications system include: wavelength division multiplexing in U.S. Pat. Nos. 4,700,339 and 5,767,998; and minimizing soliton-soliton interactions so as to increase bandwidth in U.S. Pat. No. 5,684,615. All of this prior art concerns fiberguides ( round optical waveguides or optical fibers). Indeed, the design of the fiber aspects of a communications system have reached a high level of sophistication (Hasegawa and Kodama, Solitons in Optical Communications, Claredon Press (1995)).
Inputting, and often outputting too, of digital signals to and from optical fiber transmission lines generally requires that the signals be processed in some way. Examples of signal processing include power splitting of the signal, adding a signal to an existing bit stream or extracting a desired signal from an existing bit stream. Optical circuits which serve these processing functions are best fabricated in planar configurations using standard fabrication procedures and techniques developed for the processing of modern electronic integrated circuits. These optical circuits, generally termed planar lightguide circuits, have as a fundamental element a channel waveguide whose function is to transmit (propagate) the optical signal throughout the circuit. It is a consequence of the fabrication procedure that a channel waveguide will have a rectangular (or square) cross section. The prior art has dealt with waveguides having circular cross sections but not rectangular ones. Soliton propagation, being strongly dependent on the geometry of the waveguide, cannot be predicted for channel waveguides by following the criteria set forth for optical fibers.
Furthermore, because the digital signals are confined within a waveguide having two small dimensions and one large dimension, studies on spatial solitons have no bearing on the problems of soliton transmission through such waveguides. Temporal solitons are the vehicle for transmitting digital signals without pulse broadening, because they do not change their shape while propagating with time. Spatial solitons, in contrast, employ nonlinearity in optical properties to stabilize a beam shape spatially in a medium with three large, or at least two large, dimensions.
The problem presented in achieving a commercially and technically successful digital optical communications system for long distance communication or high bit-rate transmission is: to design not only optical fibers with necessary dimensions and optical properties and signal power to propagate temporal solitons, i.e. sustain temporal soliton transmission (prior art), but to also provide planar lightguide circuits containing channel waveguides which can propagate temporal optical solitons and are compatible with optical fiber transmission lines. The present invention provides a solution to this latter problem.